Revert of Adds thunks for getting platform handles

mojo/platform_handle/BUILD.gn

-# Copyright 2015 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-source_set("platform_handle") {
-  sources = [
-    "platform_handle_private_thunks.cc",
-    "platform_handle_private_thunks.h",
-  ]
-
-  deps = [
-    ":defs",
-    "//third_party/mojo/src/mojo/public/c/system",
-  ]
-
-  defines = [ "PLATFORM_HANDLE_IMPLEMENTATION" ]
-}
-
-# Only targets that are registering the thunks should depend upon this.
-source_set("defs") {
-  visibility = [
-    ":platform_handle",
-    "//mojo/runner:native_application_support",
-  ]
-  sources = [
-    "native_exports.h",
-    "platform_handle_functions.h",
-  ]
-}